In accordance with some embodiments, a method for processing semiconductor wafer is provided. The method includes loading a semiconductor wafer into a chamber. The method also includes creating an exhaust flow from the chamber. The method further includes depositing a film on the semiconductor wafer by supplying a processing gas into the chamber. In addition, the method includes detecting, with a use of a gas sensor, a concentration of the processing gas in the exhaust flow and generating a detection signal according to a result of the detection. The method further includes supplying a cleaning gas into the processing chamber for a time period after the film is formed on the semiconductor wafer. The time period is determined based on the detection signal.